Re: [PATCH v4 3/3] sched: optimize migration by forcing rmb() and updating to be called once

Byungchul Park
2015-11-10T01:12:39Z

On Mon, Nov 09, 2015 at 02:29:14PM +0100, Peter Zijlstra wrote:
> On Sat, Oct 24, 2015 at 01:16:21AM +0900, byungchul.park@lge.com wrote:
> > +++ b/kernel/sched/core.c
> > @@ -1264,6 +1264,8 @@ EXPORT_SYMBOL_GPL(set_cpus_allowed_ptr);
> >
> > void set_task_cpu(struct task_struct *p, unsigned int new_cpu)
> > {
> > + unsigned int prev_cpu = task_cpu(p);
> > +
> > #ifdef CONFIG_SCHED_DEBUG
> > /*
> > * We should never call set_task_cpu() on a blocked task,
> > @@ -1289,15 +1291,14 @@ void set_task_cpu(struct task_struct *p, unsigned int new_cpu)
> > #endif
> >
> > trace_sched_migrate_task(p, new_cpu);
> > + __set_task_cpu(p, new_cpu);
> >
> > - if (task_cpu(p) != new_cpu) {
> > + if (prev_cpu != new_cpu) {
> > if (p->sched_class->migrate_task_rq)
> > - p->sched_class->migrate_task_rq(p, new_cpu);
> > + p->sched_class->migrate_task_rq(p, prev_cpu);
> > p->se.nr_migrations++;
> > perf_event_task_migrate(p);
> > }
> > -
> > - __set_task_cpu(p, new_cpu);
> > }
>
> I don't think this is safe, see the comment in __set_task_cpu(). We want
> that to be last.

I am sorry but I don't understand what you said. I checked the comment in
__set_task_cpu().

/*
* After ->cpu is set up to a new value, task_rq_lock(p, ...) can be
* successfuly executed on another CPU. We must ensure that updates of
* per-task data have been completed by this moment.
*/

Of course, ->cpu should be set up to a new value for task_rq_lock() to be
executed successfully on another CPU. Is this the case? Is there something
i missed? I think it would be ok if task->pi_lock can work correctly within
"if" statement in set_task_cpu(). Is there problem to do that?

>
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